High-Voltage Scanning for Three-Dimensional Nano-Sculpting with Focused Ion Beams
Focused Ion Beam (FIB) systems are powerful tools for nanofabrication, capable of milling, depositing, and imaging with nanometer resolution. When used for three-dimensional nano-sculpting—the creation of true 3D nanostructures like photonic crystals, plasmonic antennas, or micro-mechanical systems—the precision of the ion beam's positioning and its energy are paramount. This precision is governed by the high-voltage subsystems that control both the ion column (for beam acceleration and focusing) and the deflection plates (for scanning). The dynamic interplay of these high-voltage controls is what enables the translation of a digital 3D model into a physical nanostructure.
The ion column operates at a high acceleration voltage, typically ranging from 5 kV to 30 kV for liquid metal ion sources (LMIS) or up to 50 kV for plasma FIBs. This voltage, supplied by an ultra-stable high-voltage power supply, determines the ion energy and thus the penetration depth and sputter yield. For 3D sculpting, where features may have high aspect ratios and undercuts, the ability to vary the beam energy during the milling process is a significant advantage. Lower energies are used for fine surface detailing, while higher energies are used for rapid material removal in bulk regions. This requires a high-voltage supply capable of switching between setpoints rapidly and without hysteresis, while maintaining perfect focus.
The scanning of the beam is achieved by applying precise voltages to a set of electrostatic deflection plates located after the final lens. For true 3D control, the scan is not simply a 2D raster. The beam must be positioned in X and Y, but for milling deep structures with vertical sidewalls, the beam must also be tilted. This is often achieved by applying a complex, coordinated set of voltages to an octupole or other multi-pole deflector, which can both scan and tilt the beam. This requires a multi-channel high-voltage amplifier system, where each channel outputs a precisely controlled voltage (up to several hundred volts) with a bandwidth sufficient to follow the scan pattern.
The synchronization between the beam position (from the scan amplifiers) and the beam energy (from the column supply) is the key to 3D control. In a typical milling strategy, a digital 3D model is sliced into layers. For each layer, the beam is scanned in X and Y, but the beam energy may be constant across the slice. For more advanced techniques like dwell time modulation or beam energy modulation, the energy is varied pixel-by-pixel. For example, to mill a curved surface, the beam energy is reduced as the beam moves towards the edges, ensuring a constant sputter depth. This requires the high-voltage column supply to have a bandwidth comparable to the pixel rate, which can be in the megahertz range for high-speed patterning.
Beyond milling, FIBs are used for ion beam induced deposition (IBID) of metals and insulators. This process uses a gas injection system to deliver a precursor gas to the surface, which is decomposed by the ion beam. The deposition rate and the purity of the deposited material are highly sensitive to the ion energy and current density. By precisely controlling the high-voltage scanning parameters, the user can create 3D structures of arbitrary shape, such as free-standing nanowires or complex helical antennas.
Implementing this level of control requires a holistic design of the high-voltage electronics. The column supply must have sub-parts-per-million stability over the duration of a long (many-hour) sculpting run. The scan amplifiers must have exceptional linearity to avoid distortion of the pattern. The blanking amplifier, which rapidly turns the beam on and off, must have rise times in the nanoseconds to create sharp feature edges. All of these subsystems must be controlled by a central real-time computer that interprets the 3D model and generates the synchronized voltage waveforms.
This integration transforms the FIB from a simple imaging and cutting tool into a true 3D nanofabrication platform. The high-voltage power supplies are no longer separate components; they are the actuators of a 3D printer at the atomic scale, their voltages tracing the outlines of nanostructures in space, layer by layer, ion by ion. The precision of these voltages directly translates into the fidelity of the final structure, enabling the creation of devices that exploit the unique properties of matter at the nanoscale.
